1. Field of the Invention
The present invention relates to a method of controlling handoff in a cellular mobile radio communications system, and more specifically to such an arrangement and method which features a high quality speech transmission even if each of the cells is subdivided into very small sub-cells.
2. Description of the Prior Art
It is known in the art to apply frequency reuse in a mobile radio communications system in order to satisfy various objectives such as large subscriber capacity, efficient spectrum use, widespread availability, adaptability to traffic density, etc.
Frequency reuse refers to the use of the same radio carrier frequency in a number of different channels to cover different areas which are separated from one another by distances sufficient that objectionable cochannel interference does not occur. A system utilizing such concept is known as a cellular land mobile radio system.
In order to meet the ever increasing number of subscribers, subdividing each of the cells into very small ones (sub-sections) has been proposed. If one cell is subdivided into a plurality of very small sub-sections each of which has one-tenth radius of the original one, the efficiency of frequency reuse can be increased 100 fold.
However, in such a microcellular mobile radio communications system, when a communicating mobile unit moves through a plurality of cells, undesirably frequent handoffs are inevitably encountered.
In a mobile radio communication system currently employed by NTT (Nippon Telegram and Telephone, Company), the handoff is exclusively controlled by a land site and/or a mobile telephone switching office (MTSO). However, as the number of handoffs increases, the land site and/or the MTSO requires more and more sophisticated controllers. The above-mentioned NTT system is described in a book entitled "automobile telephone system" published by Japanese Electronic Communications Association, on pages 197-200.
One approach to lessening such a burden on a land site and/or MTSO, has been proposed by Toshihito Kanai, et al in a technical research paper RC S89-37 entitled "An experimental digital cellular system for high speed handoff". This paper was published by Japanese Electronic Information & Communications Association on Oct. 25, 1989. According to this paper, a mobile unit compares an average value of received signal levels of control channels of the currently connected land site with those of a nearby land site or sites. In the event that the mobile unit detects the signal strength level of a control channel which is higher than that currently in use, the communication channel is switched over to the land site of the stronger channel. According to this proposal, the handoff control is transferred to each mobile unit and, therefore the problem inherent in the firstly mentioned prior art technique is overcome.
However, in the case where a mobile unit moves along or in the vicinity of a cell boundary, the frequency of handoffs increases to such an extent that the transmission processing capacity of each land sites, MTSOs and/or exchanges tends to be exceeded. Further, each of the handoffs causes an instantaneous transmission cut-off which degrades a transmission quality.
Accordingly, in order to lower the number of handoffs to a reasonable level, it is necessary to require that certain conditions be met before the handoff can be made. According to one known technique, either one of the mobile and land sites is arranged to be able to detect the difference between the received signal levels by subtracting the signal level of a currently used channel from the signal level of a control channel of a nearby land site. Only when the difference reaches a predetermined threshold level, a handoff is permitted and thus the frequency of handoffs can be attenuated. However, if the threshold level is set too low, the frequency of the handoffs is not reduced while on the other hand, if the threshold level is set too high, a mobile unit is not "handed off" to another cell even when it is necessary.
When a time duration for detecting an average signal level is constant, the measuring error increases with the decrease in moving speed of a mobile unit and vice versa. Therefore, the above-mentioned threshold level should be raised when the mobile unit moves slowly, and should be lowered in the case of a mobile unit is moving at a high speed. Accordingly, a high threshold level should be set to a hand-held mobile terminal, while a low threshold level to a vehicle mounted mobile terminal. However, in the case where a hand-held mobile terminal is used within a vehicle compartment or a train travelling at high speed, or in the case where a vehicle mounted mobile terminal is used during a traffic jam, the above-mentioned setting of the threshold levels is no longer able to reduce the undesirable frequency of the handoffs. As a consequence, the above-mentioned problems inherent in the prior art techniques are unable to be overcome.
In connection with a vehicle mounted mobile terminal, it is possible to vary the threshold level according to a vehicle speed detected. However, there is a practical difficulty that a vehicle should be modified to enable the vehicle speed data to be supplied to the mobile terminal. Furthermore, detecting the moving speed is impossible in the case of a hand-held mobile terminal.